I'm pretty sure that GMT is a colloquial expression for Universal Coordinated Time
More historical significance* than colloquial (according to Kernighan and Ritchie in their book "The C Programming Language" about C and the C standard runtime library). That's why C has a function called
gmtime that returns a time value in Coordinated Universal Time.
It is true that GMT/UTC doesn't vary (although the time in Britain changes from it by an hour in the Summer - we call it BST for British Summer Time)
* The historical significance is that us Brits invented a standardised time first and it took the rest of the world a while to catch up
The Tigers roared in Dublin - and I was there.